The present invention relates to non-contact, optical determination of the temperature of a body. More particularly, the present invention relates to a method and apparatus for non-contact, optical determination of the temperature of a wafer of semiconductor material during processing, for the manufacturing of integrated circuits. A review of the need for wafer temperature monitoring, during processing, can be found in Graham Jackson, Yael Baharav and Yaron Ish-Shalom, "The use of temperature monitoring in advanced semiconductor industry processing", Business Briefing of the Association of South East Asian Nations: Semiconductor Manufacturing Technology, pp. 93-96, 1998.
Pyrometry is a well-known non-contact method of determining the temperature of a body such as a semiconductor wafer undergoing processing. As is well known, pyrometry infers the temperature of a body from the intensity of the electromagnetic radiation emitted by the body at different wavelengths (self-emission). According to Planck's radiation formula, the intensity of the radiation E.sub.b.lambda. d.lambda. emitted by an ideal blackbody in the wavelength band between wavelength .lambda. and wavelength .lambda.+d.lambda. is given by: ##EQU1##
where h is Planck's constant, c is the speed of light, k is Boltzmann's constant, and T is the temperature of the blackbody. In the case of a real body, equation (1) must be modified as follows: ##EQU2##
where .epsilon.(.lambda.) is the (usually wavelength-dependent) emissivity of the body. For an ideal blackbody, .epsilon.(.lambda.) is identically equal to 1 at all wavelengths. For an opaque (zero transmittance) body, the emissivity and the reflectivity R are related by a special case of the law of energy conservation, in combination with Kirchoff's law that states that the absorptivity of the body and the emissivity of the body are equal: EQU .epsilon.(.lambda.)+R(.lambda.)=1 (3)
It should be noted that equations (1) through (3) hold for each wavelength separately. This is important for the discussion below about emissivity-compensated temperature measurements of bodies with wavelength-dependent emissivity.
With regard to the angular distribution of the emitted radiation, and the behavior of .epsilon.(.lambda.) and R(.lambda.) as functions of angle:
(i) in the case of a blackbody, E.sub.b.lambda. d.lambda. is Lambertian: the radiant intensity per unit steradian is proportional to cos(.theta.), where .theta. is the angle between the line of sight and the normal to the radiating surface; PA0 (ii) equation (2) expresses the total intensity integrated over all solid angles, whereas the angular dependence of E.sub..lambda. d.lambda. depends on the physical properties of the radiating object; PA0 (iii) equation (3) is valid for two different situations, that in which .epsilon.(.lambda.) and R(.lambda.) are integrated over an arbitrary solid angle, and that in which .epsilon.(.lambda.) and R(.lambda.) are measured in a specific direction, as long as both .epsilon.(.lambda.) and R(.lambda.) are integrated over the same solid angle, or as long as .epsilon.(.lambda.) and R(.lambda.) are measured in the same direction.
In active pyrometry, the emissivity of an opaque body is determined by directing radiation of a known intensity at the body, receiving reflected radiation from the body, inferring the reflectivity of the body from the intensities of the incident and reflected radiations, and subtracting the reflectivity from 1 to obtain the emissivity. A representative patent in this field is Patton, U. S. Pat. No. 5,029,117, which is incorporated by reference for all purposes as if fully set forth herein. Patton measures the temperature of a semiconductor wafer by directing radiation from a source at the back side of the wafer, and passing light reflected and emitted by the wafer to a detector via a rotating slotted disc. The detector produces signals that are alternately representative of the intensity of combined reflected and emitted radiation, representative of the intensity of emitted radiation only, and representative of background. From these signals, the temperature of the wafer is inferred.
It is well known that, if a body has a known emissivity, which is constant as a function of wavelength but which may or may not vary as a function of temperature, then a radiometric measurement of self-emission in one single wide wavelength band provides enough data to accurately derive the temperature of the body. However, if the emissivity of the body is a function of wavelength, then the temperature can be accurately derived from self-emission measurements only if the signals are acquired in one or more narrow wavelength bands. (One narrow wavelength band suffices if the emissivity at that wavelength is known; otherwise, two or more narrow wavelengths bands are needed.) This is because a wide band measurement gives a signal which is proportional to the integral of the product of two wavelength-dependent functions, the emissivity and the blackbody Planck function of equation (2), and as a result, there is in general no unique correspondence between the self-emission signal and the temperature of the measured object. Pyrometric methods such as Patton's do not use narrow wavelength bands, and so are suboptimal for measuring the temperature of semiconductor wafers undergoing processing.
The emissivity of a silicon wafer undergoing processing is often a strong function of wavelength. FIG. 1 shows the emissivity of a silicon wafer with two layers, polysilicon above silicon dioxide, deposited on the silicon substrate of the wafer, as a function of wavelength. The thickness of the silicon dioxide layer is 1000 .ANG.. Five different thicknesses of polysilicon are shown, as indicated. This wavelength dependence degrades the accuracy of the temperature measurement.
This variation of emissivity with wavelength has been addressed by "multi-wavelength pyrometry", most commonly by a special case thereof, dual wavelength pyrometry. Stein, in U. S. Pat. No. 4,708,493, uses a dichroic beam splitter to gather reflected radiation and emitted radiation in two identical narrow wavelength bands. The two reflection signals from two separate diode lasers are used to estimate the emissivity of a body in the same wavelength bands. These emissivity values are then used together with the self-emission signals in the same bands to derive the temperature of the body. Gat et al., in U. S. Pat. No. 5,114,242, and Glazman, in WO 97/11340, obtain temperature and emissivity in a self-consistent manner from measurements of emitted radiation in several wavelength bands. These and similar methods require relatively complicated optical systems.
In principle, it is preferable to measure the emissivity directly at the same wavelength as the self-emitted radiation is measured. However, application considerations (specific production processes require different working temperature ranges) and engineering considerations (commercially available radiation sources and detectors yielding appropriate signal to noise ratio) may dictate that the emissivity be measured in different wavelength bands than the one in which the self-emitted radiation is measured. In general, as part of the design considerations, there is also the need to measure the self-emission from the same area on the wafer as the emissivity is measured (for the emissivity correction mentioned above), in order to avoid errors associated with non-uniformity of the thickness of films deposited on a semiconductor wafer, as a consequence of the dependence of the emissivity on the film thickness, as illustrated in FIG. 1.
For the past several years, C. I. Systems Ltd., of Migdal HaEmek, Israel, has been developing and selling systems for measuring the temperatures of semiconductor wafers during processing. Their first product, the NTM1, is an instrument that measures the temperature by using the temperature dependence of the absorption edge of the indirect band gap of silicon. This method is described in Michael E. Adel, Yaron Ish-Shalom, Shmuel Mangan, Dario Cabib, and Haim Gilboa, "Noncontact temperature monitoring of semiconductors by optical edge sensing", Advanced Techniques for Integrated Circuit Processing II, SPIE Vol. 1803, pp. 290-298, 1993; J. A. Roth, J.-J. Dubray, D. H. Chow, P. D. Brewer and G. L. Olson, "Feedback control of substrate temperature and film composition during MBE growth of lattice-matched InGaAs on InP", Invited Talk at 9.sub.th Conference on InP and Related Materials, IPRM '97, Hyannis, M A, May 11-15, 1997; T. J. de Lyon, J. A. Roth, and D. H. Chow, "Substrate temperature measurement by absorption-edge spectroscopy during molecular beam epitaxy of narrow-band gap semiconductor films", J. Vac. Sci. Technol. B, Vol. 15 No. 2, pp. 329-336, March/April 1997; and J. A. Roth, T. J. de Lyon and M. E. Adel, "In-situ substrate temperature measurement during MBE by band-edge reflection spectroscopy, Mat. Res. Soc. Symp. Proc., Vol. 324, pp. 353-358, 1994.
Absorption edge temperature sensing, as implemented in the NTM1, has, among its other advantages, the advantage that it exploits a phenomenon related to the shape of the spectral reflectance curve. Because this method is not based on the intensity of self-emitted radiation, the NTM1 can measure the temperatures of wafers processed at lower temperatures, and its accuracy is less sensitive than pyrometric methods to absolute measurement of the intensity of radiation. However, the method has disadvantages, such as the fact that interference effects, due to multiple layers of materials of different types deposited on the wafer, tend to distort or wash out the absorption edge effect, effectively reducing the accuracy and the temperature range of the measurement. In addition, absorption edge sensing is not suitable for certain types of wafers, for example highly doped wafers, because these do not have an absorption edge. Obviously, it is desirable that an in-situ temperature monitor, suitable for the production environment, be able to function with all or almost all types of wafers. A version of the NTM1 with an additional self-emission measurement channel has been developed, to include highly doped wafers in its repertoire, but the final instrument cost is too high for production applications, due to the need for spectral measurements.
In order to expand the population of wafers that can be measured, and improve the measurement accuracy and repeatability, C. I. Systems Ltd. introduced the NTM5, a second-generation non-contact, in-situ temperature monitor for wafer processing, based on the measurement of the wafer's self-emission. With the NTM5, C. I. Systems Ltd. introduced the concept of "radiance contrast tracking" (RCT), augmented by emissivity compensation. This method is described in U.S. Pat. No. 5,823,681, to Cabib et al., which is incorporated by reference for all purposes as if fully set forth herein, and in Michael E. Adel , Shmuel Mangan and Yaron Ish-Shalom, "Emissivity compensated, radiance contrast tracking pyrometry for semiconductor processing", Microelectronic Processors Sensors, SPIE Proceedings, Vol. 2091, pp. 311-322, 1993.
Radiance contrast tracking is based on the observation that the blackbody Planck function versus wavelength has a maximum that shifts t o short wavelengths as the temperature rises. As a result, different temperature ranges can be measured more efficiently in different wavelength ranges. Furthermore, the errors introduced in the wafer temperature measurement, by factors such as: wafer emissivity, background radiation, and inherent detector and electronic noise, are also wavelength dependent. As a result, in order to be able to measure wafer temperature in a relatively wide temperature range (about 100 C to 600 C) with the required accuracy, the NTM5 was designed to measure at several wavelengths simultaneously, by means of several detectors, that are sensitive in different regions of the spectrum, in a sandwich configuration. In cases where large wafer to wafer emissivity variation is expected, the system is augmented by a wafer emissivity compensation station, which allows emissivity compensation of each wafer by reflection and transmission spectroscopy. Application of the NTM5 to wafer temperature monitoring during Physical Vapor Deposition (PVD) is described in Michael E. Adel, Shmuel Mangan, Howard Grunes, and Vijay Parkhe, "True wafer temperature during metallization in physical vapor deposition cluster tools", SPIE Vol. 2336, pp. 217-226, 1994.
The NTM5 also addresses another limitation of prior art methods in which the light is received by the detector from the wafer over only a limited range of solid angles, a limitation that makes the measurement sensitive to superficial roughness of the wafer. There are two optical configurations that compensate for superficial roughness: one in which the wafer is illuminated hemispherically (solid angle of 2.pi. steradians) by the incident radiation, and in which the reflected radiation and the self-emitted radiation are collected from the same solid angle, and another in which the wafer is illuminated from any solid angle and the reflected and self-emitted radiations are collected hemispherically. In practice, it suffices to illuminate or receive radiation in a solid angle of a few steradians. Such a method is implemented in the NTM5, as described in U. S. Pat. No. 5,823,681. The optics of the wafer emissivity compensation station, and of the measurement probes that are used to measure temperature during processing, are designed to have identical, relatively large solid acceptance angles.
The NTM5 has the following disadvantages:
1. The need for an emissivity compensation station, which is a separate chamber where the reflectivity and transmittance are measured, for the estimation of the emissivity of each wafer, makes measurements with the NTM5 cumbersome, time consuming, and expensive.
2. The emissivity is temperature dependent, and therefore large inaccuracies may be introduced by the fact that the temperature of the wafer during processing is different th an the one in the emissivity compensation station.
There is thus a widely recognized need for, and it would be highly advantageous to have a method and apparatus for accurate, non-contact wafer temperature measurement, that can cope with strong variations of emissivity with wavelength and temperature, and with wafer surface roughness, and at the same time is compact, uses the minimum number of optical components, saves measurement time by avoiding a separate calibration chamber for emissivity compensation, does not need any moving parts, is sufficiently flexible to be adapted to work in different types of wafer production processes, and measures a large number of wafer types.